Piston for swash plate pump

ABSTRACT

A piston assembly for a hydraulic swash-plate pump is provided with a floating bearing which compensates for lateral flexure of the piston to achieve substantially total contact with the cylinder wall regardless of piston flexure and thus minimize the tendency of the piston to wear the open end portion of the cylinder out of round.

United States Patent 1191 1111 3,828,657 Neuman Aug. 13, 1974 [5 PISTONFOR SWASl-l PLATE PUMP 2,068,859 l/l937 Jones et al. 92/258 2,460,9482/l949 Sander 92/258 [75] lnvemorw" Odde" Valley 2,847,261 8/1958Cornelius 92/179 3,36l,040 l/l968 Chenault 92/258 X [73] Assignee: FMCCorporation, San Jose, Calif.

Primary Examiner-Irwin C. Cohen 1 Flledi June 29, 1972 v Attorney,Agent, or Firm-R. s. Kelly; c. E. Tripp [21] Appl. No.: 267,299

, [57] ABSTRACT 52 us. Cl 92/258, 91/488, 92/172 A Piston assembly for ahydraulic swash-Plate p p is 51 Int. Cl Fl6j 1/00 provided with afloating bearing which compensates 5 Field f Search 92 172 79 255 257for lateral flexure of the piston to achieve substantially 92 253; 91 43total contact with the cylinder wall regardless of piston flexure andthus minimize the tendency of the pis- 5 References Cited ton to wearthe open end portion of the cylinder out UNITED STATES PATENTS 680,4658/1901 Reynolds 92/258 6 Claims, 3 Drawing Figures .OOlS-DO l I l 9 4.0%

PISTON FOR SWASII PLATE PUMP BACKGROUND OF THE INVENTION The field ofthe present invention concerns hydraulic swash-plate pumps, eitherdriving or driven, which for convenience may be respectively referred toas pumps and motors. Prior art pumps of the swash plate type are usedextensively in many heavy duty applications and, due to their mode ofoperation, have a universal wear problem concerning the pistons andcylinder bores.

More specifically, a typical swash-plate apparatus will include a shaftcarrying a cylinder block. A series of spaced pistons operate inparallel relationship to the shaft within cylinder bores in the cylinderblock. Each piston has a ball end engaged in the ball socket of anassociated bearing shoe that slides over the flat face of anon-rotatable swash plate, the face being in oblique relation to theshaft. Thus, rotation of the cylinder block causes axial movement of thepistons.

In the case of a pump, the shaft will be driven ro rotate the cylinderblock. In the case of a motor, fluid under pressure is admitted to thecylinders to cause the pistons to react against the swash plate androtate the cylinder block and shaft. In either case, rotary movement ofthe shaft either develops from, or results in, relatively large lateralforces on the pistons, and tends to flex each piston and misalign itrelative to its cylinder bore. The end result is that each of thepistons will cause appreciable wear, both to the piston and the cylinder wall, at the end of its associated cylinder bore where the lateralload is concentrated due to the cocking of the piston.

There are obvious ways to inhibit this wear problem, such as byincreasing the physical size of the compo nents to resist suchflexurefor by choosing structural alloys which will accomplish a similarobjective. It is readily seen, however, that these precautions cannotmeet all conditions such as when the structure must be so heavily orexpensively constructed that specified cost or weight and sizerequirements are exceeded.

According to the present invention, there is provided a simple andinexpensive solution of universal application to hydraulic swash platepumps for the reduction of the usual wear which tends to make thepistons and cylinders out of round at the open end areas of thecylinders.

SUMMARY OF THE INVENTION A pivotable bearing sleeve is mounted on thepiston of a hydraulic swash plate pump at about the point where maximumextension of the piston will position the sleeve within the end portionof the cylinder bore. By the provision of means for pivotally mountingthe sleeve to the piston and two spaced cylinder contacting surfaces forthe piston, localized loading of the piston is avoided and the bearingsleeve retains substantially total contact with the cylinder surface todistribute the lateral load from the piston over a relatively widebearing area of the cylinder. Since the piston is stressed duringoperation so that its axis becomes non-linear, the capability of thebearing sleeve to maintain contact with the piston and to maintainsubstantially coextensive contact with the cylinder surface results inminimal lateral thrust wear of both the piston and the cylinder.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic longitudinalsection through a typical hydrostatic transmission of the variable pumpdisplacement, fixed motor displacement type.

FIG. 2 is an enlarged schematic section through a piston and bearingshoe of the motor section of the FIG. I apparatus.

FIG. 3 is a schematic section similar to FIG. 2 wherein the stressforces are exaggerated to illustrate the operational principles of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT One type of known swash plateoperated hydrostatic transmission 10 is schematically illustrated inFIG. 1, and includes, within a bolted housing 12, a variabledisplacement hydraulic pump P and a hydraulic motor M. A power inputshaft 18 drives the pump P, and a power output shaft 20 is driven by themotor M. The operating fluid which is pumped between the pump P andmotor M is also employed for lubrication.

A cylinder block 22 is secured to the power input shaft 18 and isprovided, for example, with nine or some other odd number of cylinderbores 24, only one of which is shown. Each cylinder bore is providedwith apiston 26 having a ball end 28 projecting from the cylinder block22 and being engaged in the complementary socket of a bearing shoe 30.Each bearing shoe slides across a flat face of a non-rotatable swashplate 34 that circumscribes the power input shaft 18. As illustrated,the face of the swash plate 34 is oriented in a plane normal to theshaft 18 and will not, therefore, axially move the pistons 26 as theyare rotated by the shaft. This position of the swash plate thuscorresponds to a zero output condition of the pump P.

The swash plate 34 can be tiltedobliquely of the axis of the input shaft18 by swash plate tilt mechanisms 36, whereby rotation of the cylinderblock 22 results in axial movement of each piston 26 and pumping ofhydraulic fluidat a rate corresponding to the strokeof the pistons asgoverned by the tilt of the swash plate 34. Fluid is discharged fromeach pump cylinder through a port 40 which registers with valve ports,not shown,

in a fixed valve plate 42 as the cylinder block 22 rotates.

Similar components are used in the motor M, and include a cylinder block52 fixed to the power output shaft 20. A series of circumferentiallyspaced cylinder bores 54 (only one of which is shown) are each providedwith a piston 56 and a port 58. At the projecting end of the piston, aball connector 60 is engaged with a complementary socket ina bearingshoe 62. To energize the motor, the fluid discharged under pressure fromthe pump P is sequentially routed by the valve plate 42 into the ports58 to axially force the bearing shoes 62 against a fixed, inclined swashplate 64. The reaction forces thus developed produce a lateral thrustwhich is delivered to the bearing shoes, pistons and cylinder block torotate the power output shaft 20 with a rotational torque governed bythe displacement of the pump P, the operating pressure, frictionallosses and other known parameters.

It can be readily appreciated that swash plate pumps and motors, due totheir inherent operating mode, develop appreciable dynamic bendingmoments in the pistons, especially at maximum stroke. Thus, the piston,in an exaggerated and oversimplified sense, is comparable to acantilevered beam having a concentrated load near its ball end. Ofcourse, the actual kinematics of rotation, as related to the pistons, isa complex design study, but the net effect in any case is that thepistons exert considerable lateral force upon their respectivecylinders, and tend to wear the pistons and wear the cylinders out ofround at the open ends thereof where the lateral loads are concentrated.There are obviously many ways of bringing this wear within acceptablelimits, such as by the selection of certain alloys for the cylinders andpistons which will more effectively combat wear, or to increase thecross sectional area of the pistons so that they will be more resistantto bending. For reasons of cost, compliance with accepted designfactors, meeting a specified efficiency, or for other reasons, noexpedient has been found to be entirely satisfactory to overcome thestated wear problem.

In accordance with the present invention, there is an inexpensive,simple and highly efficient solution to the problem of accommodating thelateral thrust in the pistons and cylinders of a swash plate pump ormotor. Basically, the invention provides a bearing sleeve 70 (FIG. 2)which is mounted in substantially fixed position on the piston (eitherpiston 26 or 56 in FIG. 1) but is free to rock about a transverse axisby a limited amount.

With more specific reference to FIG. 2, the piston 56 comprises acylindrical inner body 72 which at one end includes the integral ballconnector 60. At its other end, the piston body 72 is counter-bored toprovide an annular flange 74 that is swaged against a recess portion 76of an end cap 78 that forms the head of the piston. An axial bleedpassage 80 throughv the body 72 transfers part of the operating fluidapplied-against the head of the piston to the ball connector 60, thebearing shoe 62, and the swash plate 64 for lubrication purposes. Itshould here be noted that .the specific details of the axial passage 80,the bearing shoe 62 and the particular swaged connection of the body 72to the end cap, orpiston head, 78 are not critical to an understandingof the present invention.

Near the ball connector 60, the piston body 72 is formed with an annularflange 82 and-an adjacent annular land 84. The flange 82 provides foraxial retention, with limited endwise movement, of the bearing sleeve70. The land 84 provides a journal surface for fulcrum engagement of thepiston body with the bearing sleeve 70, and has a specific radialclearance from the bearing sleeve in order to permit relative pivotablemovement between the piston and the bearing sleeve, as will be presentlyexplained. A tubular spacer sleeve 88 circumscribes the body 72 andextends between the confronting surfaces of the bearing sleeve 70 andthe end cap 78, but with a small end clearance which allows verylimitedaxial movement of the spacer sleeve 88. The spacer sleeve 88 has nocontact with the cylinder 54.

The dynamic flexure of the piston 56 can be calculated, or estimated,within reasonably close limits. Upon this basis, the aforementionedoperating clearances can be determined in order to allow, or at leastnot inhibit, such flexure, but at the same time maintain the exteriorsurface of the sleeve bearing 70 in substantially coextensive contactwith the surface of the cylinder 54. Thus, in one specific example, theexternal clearance between the land 84 and the interior surface of thebearing sleeve 70, the cylindrical space indicated by the referencenumeral 90, was the usual 0.00l5 to 0.0020 inches clearance toaccommodate flexure of the piston without tilting the bearing sleeve. Itwas determined that the running clearance at 92 between the exteriorsurface of the bearing sleeve and'the interior surface of the cylinder54 should be in the order of 0.0013 inches for maintaining a film oflubricating fluid. Because the land 84 extends radially outward from thebody of the piston, a pair of cylindrical recesses 94 and 95 areprovided at the ends of the bearing sleeve. The recess 94 at the outerend of the sleeve is large enough (in practice about 0.015 inches) toaccommodate any bending of the piston without permitting contact betweenthe piston and bearing sleeve. Thus, all load transfer between thepiston and the bearing sleeve is accomplished through the land 84, andit is desirable to position the land as close to the center 'of thesleeve as possible so that the load will be transferred uniformlybetween the sleeve and the surrounding cylinder wall over the entirecontacting surface of the sleeve. Because the flange 82 on the body 72will tilt relative to the bearing sleeve 70, the spacer sleeve 88 shouldhave a clearance at one end. This clearance is here illustrated at 93(FIG. 2) and is about 0.002 inches. The result of the variousaforedescribed operating clearances is best understood from aconsideration of FIG. 3.

FIG. 3 illustrates the same structure as FIG. 2, but the flexures andoperating clearances are further exaggerated to more clearly disclosethe mode of operation. One important point to note is that the pistonassembly contacts the cylinder bore only via the end cap, or pistonhead, 78 and the bearing sleeve 70. This fact, and the fact that thebearing sleeve 70 lies approximately midway between the ball connector60 and the bearing cap 78, allows a generally coextensive flexure of thepiston, rather than concentrating the flexure near the bearing sleeve70.

Because the land 84 is in centered relation to the bearing sleeve 70, isaxially shorter than the bearing sleeve, and has the operating clearanceat 90, the land 84 can tilt relatively to the bearing sleeve and stillmaintain substantial contact therewith, while the bearing sleevemaintains coextensive contact with the cylinder 54. Stated otherwise,the overall flexure of the piston body 72 can be substantially greaterthan a localized flexure at the land 84, and yet the bearing sleeve 70,because of its pivotal connection with the land, will not only maintaincoextensive contact with the cylinder bore but will be isolated andunaffected by such flexure with the result that there is less than theusual tendency for wear of the piston andcylinder at the open end of thecylinder.

As the piston is retracted and the bearing sleeve 70 moves inward in thecylinder, the same degree of piston flexure can be accommodated sinceonly the bearing sleeve 70 and the end cap 78 have sliding contact withthe cylinder. At the same time, the spaced bearing sleeve and end capallow the dynamic bending stress of the piston to take placethroughoutan extended length of the piston, thereby minimizing localized flexurein the areaof the bearing sleeve 70. Meanwhile, the bearing sleeve 70provides for improved load distribution between the piston and cylinder,and prevents the generation of high loads at the edge of the cylinderbore to prolong the useful service life of the piston and cylinder.

Although the best mode contemplated for carrying out the presentinvention has been herein shown and described, it will be apparent thatmodification and variation may be made without departing from what isregarded to be the subject matter of the invention.

What is claimed is:

1. In a swash plate pump including a cylinder, the improvementcomprising a piston assembly for reception in said cylinder in order tominimize lateral dynamic thrust wear of the piston and cylinder, saidassembly comprising an elongate piston body having an enlarged pistonhead for sliding contact with the cylinder, and a cylindrical bearingsleeve mounted on said piston body in axial spaced relation to saidpiston head for sliding contact with the adjacent portion of thecylinder, means for providing a fulcrum engagement between said bearingsleeve and said piston body to allow overall flexure of said piston bodywhile maintaining said bearing sleeve substantially coaxial relative tothe bore surface of said cylinder so that the load from the piston isdistributed substantially uniformly over the external bearing area ofthe sleeve, a predetermined radial clearance being provided between thepiston body and the sleeve in order to permit relative pivotal movementtherebetween, said means for providing a fulcrum engagement comprisingan enlarged land portion on said piston body adjacent said sleeve forcontact with the central portion of said sleeve upon lateral movement ofsaid piston body taking up said radial clearance, said land portionhaving an axial length less than the axial length of the sleeve so thatlateral loads on said piston are transferred to said sleeve within thecentral portion of the sleeve.

2. Apparatus according to claim 1 including a radial flange on saidpiston body in endwise blocking relation with said bearing sleeve, and atubular spacer mounted on said piston body between said bearing sleeveand said piston head to limit the axial displacement of said bearingsleeve.

3. Apparatus according to claim 2 wherein said spacer is provided with adiameter less than the cylinder to accommodate flexure of said pistonbody.

4. Apparatus according to claim 3 wherein said spacer has an end-to-enddimension providing a clearance between said spacer and said piston headand allowing a predetermined axial movement of the spacer between saidbearing sleeve and said piston head.

5. Apparatus according to claim 4 wherein said clearance isapproximately 0.002 inches, the radial clearance of said bearing sleevewith said enlarged land portion of the piston body being approximately0.0015 to 0.0020 inches.

6. In a swash plate pump, a piston assembly comprising an elongatepiston body, a cylindrical end cap secured to said body and forming thehead of the piston, a projecting radial flange on said body in axiallyspaced relation to said piston head, and a bearing sleeve and a tubularspacer mounted on said body intermediate said flange and said pistonhead, said bearing sleeve having an external diameter equal to that ofsaid piston head and said spacer having a lesser diameter, said bearingsleeve having limited radial clearance with said piston body in theorder of 0.0015 to 0.0020 inches, and said piston having an enlargedportion adjacent said sleeve for contact with said sleeve within thecentral portion thereof upon lateral movement of said piston body takingup said radial clearance so that lateral loads on said piston will bedistributed substantially uniformly throughout the external bearingsurface of

1. In a swash plate pump including a cylinder, the improvementcomprising a piston assembly for reception in said cylinder in order tominimize lateral dynamic thrust wear of the piston and cylinder, saidassembly comprising an elongate piston body having an enlarged pistonhead for sliding contact with the cylinder, and a cylindrical bearingsleeve mounted on said piston body in axial spaced relation to saidpiston head for sliding contact with the adjacent portion of thecylinder, means for providing a fulcrum engagement between said bearingsleeve and said piston body to allow overall flexure of said piston bodywhile maintaining said bearing sleeve substantially coaxial relative tothe bore surface of said cylinder so that the load from the piston isdistributed substantially uniformly over the external bearing area ofthe sleeve, a predetermined radial clearance being provided between thepiston body and the sleeve in order to permit relative pivotal movementtherebetween, said means for providing a fulcrum engagement comprisingan enlarged land portion on said piston body adjacent said sleeve forcontact with the central portion of said sleeve upon lateral movement ofsaid piston body taking up said radial cLearance, said land portionhaving an axial length less than the axial length of the sleeve so thatlateral loads on said piston are transferred to said sleeve within thecentral portion of the sleeve.
 2. Apparatus according to claim 1including a radial flange on said piston body in endwise blockingrelation with said bearing sleeve, and a tubular spacer mounted on saidpiston body between said bearing sleeve and said piston head to limitthe axial displacement of said bearing sleeve.
 3. Apparatus according toclaim 2 wherein said spacer is provided with a diameter less than thecylinder to accommodate flexure of said piston body.
 4. Apparatusaccording to claim 3 wherein said spacer has an end-to-end dimensionproviding a clearance between said spacer and said piston head andallowing a predetermined axial movement of the spacer between saidbearing sleeve and said piston head.
 5. Apparatus according to claim 4wherein said clearance is approximately 0.002 inches, the radialclearance of said bearing sleeve with said enlarged land portion of thepiston body being approximately 0.0015 to 0.0020 inches.
 6. In a swashplate pump, a piston assembly comprising an elongate piston body, acylindrical end cap secured to said body and forming the head of thepiston, a projecting radial flange on said body in axially spacedrelation to said piston head, and a bearing sleeve and a tubular spacermounted on said body intermediate said flange and said piston head, saidbearing sleeve having an external diameter equal to that of said pistonhead and said spacer having a lesser diameter, said bearing sleevehaving limited radial clearance with said piston body in the order of0.0015 to 0.0020 inches, and said piston having an enlarged portionadjacent said sleeve for contact with said sleeve within the centralportion thereof upon lateral movement of said piston body taking up saidradial clearance so that lateral loads on said piston will bedistributed substantially uniformly throughout the external bearingsurface of the sleeve.